The present invention relates to an induction heating unit used as a heat source of fixing device in an image forming apparatus such as copying machine, printer, FAX and complex machine thereof, particularly to a coil heating structure. The present invention also relates to a fixing device using the induction heating unit and a method for attaching coil for the induction heating unit.
Conventionally, a heating roller system using a halogen heater has been adopted as a heat source for fixing device of image forming apparatus. From an energy-conservation point of view, an induction heating system having a high heat transfer efficiency has been focused on and developed for practical use. In the fixing device using the induction heating system, a heat transfer efficiency same as or higher than the heating roller system is obtained so that the device can be quickly started up. Especially, an induction heating unit in which magnetic flux generated by coil is directed to heating layer through core material such as ferrite is compact and efficiency.
The fixing device has a fixing roller and a press roller which surfaces come into contact with each other and are rotatable. The fixing roller comprises at least five layers of in order from inside support layer, sponge layer, electromagnetic induction heating layer, resilient layer, and releasing layer. Due to magnetic flux generated by a magnetic flux generating means provided around the fixing roller, the electromagnetic induction heating layer of the fixing roller produces heat. When a recording material with an unfixed toner image supported is conveyed and sandwiched in the nip portion between the fixing roller and the press roller, the unfixed toner image on the recording material is melted due to the heat of the heated electromagnetic induction heating layer and fixed to the recording material.
In such fixing device of electromagnetic induction heating system, in order to ensure quality of fixation, it is required to heat the member to be heated in uniform distribution of temperature and rise the temperature at high speed. However, if heat capacity of the member to be heated is made lower in order to obtain a high speed temperature rising property, it would be difficult to ensure a uniformity of the temperature distribution. In order to uniform the temperature distribution, it is necessary to maintain a distance between the coil and the member to be heated with a high degree of accuracy.
As a bobbin to which the coil is attached is metal molded, the geometry of the coil attaching surface has high accuracy. So the distance between the coil attaching surface of the bobbin and the member to be heated can be maintained in a high degree of accuracy. Therefore, it is important to attaching the coil on the coil attaching surface.
As a method for attaching the coil on the bobbin, as shown in FIG. 13, there has been a method of directly attaching coil 54 on the coil attaching surface of a bobbin 53 disposed outside of a fixing belt 52 which is provided around a heating roller 51. In this method, loose of the coil 54 makes it difficult to ensure the accuracy of the distance between the coil 54 and the heating roller 51 that is a member to be heated. Further, thermal expansion at high temperature and vibration causes the coil 54 to shift and flip-flop. Especially, in the case that the unit is small and winding number of the coil 54 is low and that the coil 54 is attached in multiple stages, it is difficult to ensure the distance between the coil 54 and the heating roller 51 and also uniform the temperature distribution of the heating roller 51.
As shown in FIG. 14, there has been also a method comprising steps of bonding coil 54 to a bobbin 53 with a silicon adhesive tape 55 and winding PFA tape 56 partly on the coil 54, preventing the coil 54 from raveling. However, in this method, raveling of the coil 54 of parts other than the parts on which the PFA tape 56 is wound may be caused.
The patent document 1 discloses a method in a fixing device for induction heating a fixing roller from inside, as shown in FIG. 15, the method comprising steps of directly winding coil 58 on a bobbin 57 and impregnating the coil 58 with modified silicon resin 59 to bond the coil 58 to bobbin 57, preventing the coil 58 from raveling. The patent document 2 discloses a method in a fixing device for induction heating a fixing roller from inside, the method comprising steps of forming litz wire in a coil wound shape, press forming the coil, and solidifying the coil with adhesive and so on. However, these methods have disadvantages that torsional deformation is generated in the coil due to lack of adhesion force; accuracy of the distance between the coil and the member to be heated becomes worsen; temperature distribution does not become uniform; and noise due to slack of the coil is generated.    Patent Document 1; JP2002-174971A    Patent Document 2; JP2002-373774A